This invention relates to conjugated diene butyl rubber elastomers and more particularly to the method for curing this category of elastomeric copolymers.
Conjugated diene butyl elastomers are known and are described in U.S. Pat. No. 3,775,387 issued Nov. 27, 1973 to Baldwin and U.S. Pat. 3,816,371 issued June 11, 1974 to Baldwin and Malatesta. Generally, these elastomers are copolymers consisting of from 85 to 99.5% by weight of a C.sub.4 -C.sub.7 isoolefin combined with 15 to 0.5% by weight of a conjugated diolefin having 4 to 14 carbon atoms, the copolymer containing randomly distributed sites of conjugated diene unsaturation. These elastomers are preferably prepared by dehydrohalogenation of halogenated butyl rubber.
As described in the aforesaid patents, these elastomers can be cured with sulfur and sulfur-containing vulcanizing agents or co-vulcanized with general purpose rubbers. It is also known that these CDB elastomers can be cured with polyfunctional dienophiles such as m-phenylenebis-maleimide, ethylene glycol dimethacrylate. Generally, these curing agents, except for highly reactive dienophiles, require the use of fairly high temperatures, that is, in excess of 300.degree. F., in order to promote an effective cure. The present invention provides a method wherein the cure may be carried out at room temperature or temperatures in excess of room temperature but, in either case, the rate of cure is easily controlled by modification of the cure system package through regulation of the oxidation reaction which is necessary to formation of the benzoquinone dienophile crosslinking agent.
In accordance with the present invention, there has been discovered a method for curing and crosslinking conjugated diene butyl rubber elastomers, said elastomers being copolymers consisting of 85 to 99.5% by weight of a c.sub.4 -C.sub.7 isoolefin combined with 15 to 0.5% by weight of a C.sub.4 -C.sub.14 conjugated multi-olefin, the copolymer containing randomly distributed sites of conjugated diene unsaturation, the method comprising adding to said elastomer about 1 to 6, preferably 2 to 4, parts per hundred parts by weight of 1,4-dihydroxybenzene and an effective amount of an oxidation catalyst or an oxidizing agent thereby oxidizing the dihydroxybenzene to 1,4-benzoquinone in situ with said benzoquinone curing said elastomer at room temperature or temperatures in excess of room temperature by formation of crosslinking moieties between elastomer polymer chains.
In the present invention the curing may be carried out at room temperature or temperatures in excess of room temperature and the desired curing temperature will have an effect on the proper choice of oxidizing agent or oxidation catalyst. In room temperature curing and curing at temperatures moderately above room temperature, that is, temperatures of about 60.degree. to 120.degree. F., a metal carboxylate oxidation catalyst is employed for optimum results. The oxidizing agents alone, in the absence of the oxidation catalysts, are generally not effective at these temperatures. However, at temperatures in excess of about 200.degree. F. oxidizing agents have been found to be effective with or without the presence of a carboxylate oxidation catalyst. At these moderately high temperatures, that is, about 200.degree. to 350.degree. F., satisfactory cures are obtained over periods of about 10 to 40 minutes. Conventional curing agents for CDB elastomers such as sulfur-based curing agents generally require curing temperatures having a minimum of about 290.degree. to 300.degree. F.
The overall crosslinking process may be generalized by the following equations showing oxidation of dihydroxybenzene to benzoquinone and the crosslinking of polymeric chains by the benzoquinone so formed. ##STR1##
The conjugated diene elastomeric copolymers which are cured in accordance with the present invention are known and are described in U.S. Pat. No. 3,816,371 issued June 11, 1974 to Baldwin et al, and U.S. Pat. No. 3,775,387 issued Nov. 27, 1973 to Baldwin as well as in U.S. application Ser. No. 465,479 filed by Baldwin et al, on Apr. 30, 1974, all of which are incorporated herein by reference.
These conjugated diene butyl elastomers may be generally described as copolymers consisting of from 85 to 99.5% by weight of a C.sub.4 -C.sub.7 isoolefin combined with 15 to 0.5% by weight of a conjugated diolefin having 4 to 14 carbon atoms, the copolymer containing randomly distributed sites of conjugated diene unsaturation. Moreover, as described in U.S. application Ser. No. 465,479 filed by Baldwin et al on Apr. 30, 1974, the conjugated diene functionality may be present such that both olefin units are in the backbone chain, or both are outside of the chain, or one may be inside the chain and one outside, or both may be present in a ring, such as by dehydrohalogenation of allylic halogen of cyclopentadiene. Dehydrohalogenation of a butyl-type polymer prepared from dimethylbutadiene and isobutylene can provide conjugated olefinic structures having the following configurations: ##STR2##
Preferably the copolymers useful in the present invention are copolymers of isobutylene and isoprene, a major portion of the isoprene units combined therein having conjugated diene unsaturation and may be represented by the following structure: ##STR3## where n + m represent the number of isoolefin units incorporated into the butyl rubber polymer backbone and m represents the number of conjugated diolefin units present substantially as isolated units.
The preferred method of preparing the elastomers useful in the present invention is through dehydrohalogenation of a halogenated butyl rubber.
The term "butyl rubber" is used in the industry to describe copolymers made from a reaction mixture containing 70 to 99.5% by weight of a C.sub.4 -C.sub.7 isoolefin such as isobutylene and 30 to 0.5% by weight of a C.sub.4 -C.sub.14 conjugated multiolefin such as isoprene. The resulting copolymers contain 85 to 99.5% by weight of isoolefin and 0.5 to 15% of combined multi-olefin. The polymer backbone of commercial butyl rubber consists primarily of isobutylene units with just a few percent of isoprene units, the latter contributing the small amount of unsaturation present in butyl rubber.
Halogenated butyl rubber is described in U.S. Pat. No. 3,099,644 and is typified by the following formula: ##STR4## Halogenated butyl rubber may be prepared by halogenating butyl rubber in a solution containing 1 to 60% by weight butyl rubber in a substantially inert C.sub.5 -C.sub.8 hydrocarbon solvent such as pentane, hexane, heptane and contacting the solution with halogen gas for about 2 to 25 minutes whereby halogenated copolymers form containing up to 1 or somewhat more halogen atom per double bond initially present. Illustrative is "Chlorobutyl 1068" a chlorinated butyl rubber which before halogenation contains about 1.8 mole % unsaturation and a viscosity average molecular weight of about 450,000.
The conjugated diene butyl elastomers useful in the present invention are preferably prepared via dehydrohalogenation of a halogenated butyl rubber. Generally this process comprises contacting a solution of halogenated butyl rubber with: (1) a soluble metal carboxylate where the metal is selected from the metal of Groups IIb, IVa and VIII of the Periodic Table; (2) a soluble carboxylic acid; and (3) an oxide or a hydroxide of the metal selected from Groups Ia and IIa of the Periodic Table. This dehydrohalogenation process is described in detail in U.S. Pat. No. 3,775,387 issued Nov. 27, 1973 to Baldwin et al and is incorporated herein by reference.
The mole percent of conjugated diene unsaturation in the elastomers useful in the present invention is from about 0.5 to about 2.5 and these conjugated diene butyl elastomer copolymers have a number average molecular weight generally from about 5,000 to 500,000.
Room temperature curing or curing at temperatures moderately in excess of room temperature using a metal carboxylate oxidation catalyst constitutes a preferred embodiment of the present invention. The preferred temperature range is about 60.degree. to about 120.degree. F. and the curing process is carried out over a period of about 20 to 200 hours. A heavy metal carboxylate catalyst such as vanadium, cobalt, copper or iron carboxylate is used. Generally salts of carboxylic acids having 8 to 20 carbon atoms are useful such as the naphthenates, octoates or neodecanoate. Cobalt and copper naphthenate and neodecanoate have been found particularly effective and are preferred. These catalysts are used in amounts ranging from about 0.05 to 0.25 phr (parts per hundred) parts by weight of elastomer, preferably about 0.1 phr. Room temperature curing is of particular value in that it allows for the use of the CDB elastomers in the areas of caulks, sealants and the like where ambient temperature curing is a requirement for practical utility. Rapid cures may be obtained at elevated temperatures of about 200.degree. to 350.degree. F. in relatively shorter time periods in accordance with the present invention. For example, it has been found that acceptable cures to desirable physical properties can be achieved at 10 to 30 minutes at temperatures of 250.degree. to 320.degree. F. using a suitable oxidizing agent in the presence or absence of the carboxylate catalyst. It has been found that at these elevated temperatures it is somewhat more preferable to employ only an oxidizing agent as the means for promoting the oxidation of 1,4-dihydroxybenzene to 1,4-benzoquinone, the latter compound functioning as the crosslinking dienophile moiety. However, the oxidation catalyst, when used alone, is also effective at these temperatures.
Suitable oxidizing agents for high temperature curing in accordance with the present invention include the metallic inorganic oxides such as lead dioxide, red lead (Pb.sub.3 O.sub.4), manganese dioxide and the like and the organic peroxides including the hydroperoxides and diacyl peroxides as exemplified by t-butylhydroperoxide, ethyl hydroperoxide, cumene hydroperoxide, t-pentyl hydroperoxide, dimethyl peroxide, dicumyl peroxide, cumyl methyl peroxide, t-butyldiphenylmethyl peroxide, di-t-butylperoxide, dibenzoyl peroxide, dilauroyl peroxide, and the like, generally the organic radicals of said peroxides having about 1 to 12 carbon atoms. Benzoyl peroxide and dicumyl peroxide are preferred.
The quantity of oxidizing agent is somewhat variable depending on the amount of 1,4-dihydroxybenzene present. Generally there is employed sufficient oxidizing agent on a stoichiometric equimolar basis necessary to effect the oxidation and usually a molar excess. The amount of the oxidizing agent present may be expressed as a molar ratio of about 0.5:1 to 2:1, preferably about 1:1 to 1.5:1, relative to the moles of 1,4-dihydroxybenzene in the elastomer.
The invention is further illustrated by the following examples which are not to be considered as limitative of its scope.